Semi-solid metal casting process of hypoeutectic aluminum alloys

ABSTRACT

A method for the refining of primary aluminum in hypoeutectic alloys by mixing at least two hypoeutectic alloys into a solid/semi-solid hypoeutectic slurry is described. The method provides control of the morphology, size, and distribution of primary Al in a hypoeutectic Al—Si casting by mixing a hypoeutectic Al—Si liquid with solid hypoeutectic Al—Si particles to impart desirable mechanical properties. The invention enables SSM molding of hypoeutiectic alloys without the need for secondary processing steps associated with other rheocasting processes.

FIELD OF THE INVENTION

The present invention relates generally to a process of casting metalalloys. More particularly, the present invention relates to a method ofsemi-solid metal casting of aluminum-silicon alloys.

BACKGROUND OF THE INVENTION

Conventional casting methods such as die casting, gravity permanent moldcasting, and squeeze casting have long been used for Aluminum-Silicon(Al—Si) alloys. However, where semi-solid metal (SSM) casting of Al—Sialloy materials has been involved, the conventional methods have notbeen employed successfully to date. Rheocasting and thixocasting arecasting methods that were developed in an attempt to convertconventional casting means to SSM casting. However, these SSM methodsrequire additional retrofitting to conventional casting machinery andchallenges remain in the ability to manipulate the microstructures ofprimary Al and/or Si in the cast part for improving cast performance.

Accordingly, it is desirable to provide a method of casting SSM Al—Sialloys utilizing both conventional and rheocasting means that can impartdesirable mechanical properties. In particular, there is a need for aprocess to control the nucleation of primary Al particles inhypoeutectic Al—Si alloys to limit the formation of large primary Alparticles. Further still, it is desirable to provide a method ofproducing products with Al—Si alloy castings by conventional orrheocasting techniques wherein the temperature of the semi-solid slurrycan be controlled.

SUMMARY OF THE INVENTION

The foregoing needs are met, to an extent, by the present invention,wherein according to one embodiment, an SSM casting process is providedthat generates products with Al—Si alloy castings by conventional orrheocasting techniques wherein the temperature and the final morphologyof the primary Al of the product can be controlled.

In accordance with one embodiment of the present invention an SSMcasting process is provided comprising heating a first Al—Sihypoeutectic alloy to a first temperature, combining the heated alloywith a second Al—Si hypoeutectic alloy having a second temperature toform a semi-solid metal, cooling the combined first and second Al—Sihypoeutectic alloys for a determined length of time, and then castingthe semi-solid metal. The length of cooling time can be zero. The alloysmay be of the same or different chemical composition. The alloys mayalso be heated to the same or different temperatures.

In accordance with another embodiment of the present invention an SSMcasting process is provided wherein the temperature of a first Al—Sihypoeutectic alloy is higher than the temperature of a second Al—Sihypoeutectic alloy such that there is a difference in temperaturebetween the first and second Al—Si hypoeutectic alloys. The differencein temperature may be chosen to achieve a determined rate of coolingwhich may allow control of primary Al particle size in the final castproduct. In some embodiments, hypoeutectic Al—Si cast products may haveAl particles with an average diameter ranging from about 40 microns toabout 60 microns. The difference in temperature may also be chosen toachieve a faster rate of cooling of the hotter alloy as compared toheating the hotter Al—Si hypoeutectic alloy and allowing the hotteralloy to cool independently at room temperature.

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of one embodiment of how theinventive process can be performed.

FIG. 2 shows the representative microstructure from different locationswithin a castings produced by the process of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention provides a method for controlling the composition,temperature and microstructure of Al—Si alloys prior to SSM casting inan attempt to control the mechanical properties of the final castproduct. Generally, this is accomplished by mixing at least twohypoeutectic Al—Si alloys. By definition, aluminum alloys with up to butless than about 11.7 weight percent Si are defined “hypoeutectic”,whereas those with greater than about 11.7 weight percent Si are defined“hypereutectic”. In all instances, the term “about” has beenincorporated in this disclosure to account for the inherent inaccuraciesassociated with chemical weights and measurements known and present inthe art.

The metallic composition of alloys used in current methods for SSMcasting is limited to the availability and composition of the startingmaterials. In contrast, according to the present invention, a broadrange of metallic compositions are achievable from the same startingmaterials because the combination of hypoeutectic alloys into a singularhypoeutectic alloy allows for the manipulation of the finalconcentration of Si in the Al—Si alloy by controlling the compositionand mass of the starting materials or semi-solid slurries.

Mixed hypoeutectic alloy compositions can be formed by combining two ormore aluminum alloys comprising up to but less than about 11.7 percentSi in aluminum. In one embodiment, two Al—Si alloys are combined to forma mixed hypoeutiectic alloy. It will be noted that one of the startingmaterials need not be an Al—Si alloy, but alternatively, purelyAluminum. In yet other embodiments, combinations of two or morehypoeutectic alloys with the same Al—Si chemistry (i.e., same weightpercent Si) are disclosed herein. One example of a hypoeutectic alloywith about 7% Si is developed by Elkem (under the trademark of SIBLOY®).

In addition to imparting unique physical properties to the end product,the concentration of Si in aluminum has consequences in the phaseprofile of any given alloy at any given temperature. For example,hypoeutectic Al—Si alloys begin to develop large Al particles as theybegin to cool below the liquidus and into the SSM range. In a preferredembodiment, the instant invention teaches a method of mixing two Al—Sialloys at different temperatures together so that the amount of time themixture spends in the transitional semi-solid phase is minimized.

Temperature control of the alloys can be achieved by mixing two or morehypoeutectic alloys as in the present invention. Generally, one alloy isheated to a liquid state and then mixed with an alloy of coolertemperature to bring the combined melt within the SSM range. The cooleralloy may serve as a heat sink when the hotter alloy is combinedtherewith, thus bringing the combined alloy mixture into the semi-solidregime more rapidly than using conventional coolers or air cooling. Insome embodiments, one or more of the hypoeutectic alloys is maintainedin a solid state. Preferably, the hotter or liquid alloy is generallypoured into the cooler or solid hypoeutectic alloy; however, it is alsopossible to add the cooler alloy to the hotter alloy. Solid phase alloysmay be presented in any form known in the art, which include, but arenot limited to, grains, chips, and/or pellets.

In one embodiment, when squeeze casting is involved, the alloys may beheated to a range of from about 690° C. to about 715° C. In anotherembodiment, when the SSM is refined (e.g., grain refined orelectromagnetically-stirred), the alloys may be heated typically to arange of from about 577° C. to about 580° C. In yet other embodiments,one of the alloys to be combined may not be heated at all, i.e., it maybe used at ambient room temperature.

In a preferred embodiment of the invention, a hotter alloy is combinedwith a cooler alloy, and preferably, the hotter alloy is raised to about640° C. and the cooler alloy is left at ambient or room temperature.This large temperature gradient allows for a quicker extraction of heatfrom the hotter parent alloy than with conventional coolers anddecreases the time necessary for the liquid alloy to drop in temperatureto a semi-solid/slurry processing temperature. Such rapid nucleation ofthe primary Al phase is thought to result in a more homogeneousmicrostructure throughout the material.

In this manner, the current invention can enable SSM casting ofhypoeutectic alloys via the rheocast method without secondary processingequipment such as external cooling mechanisms, or induction heatingapparatuses. For example, in one embodiment, current squeeze castingprocesses can now be converted to an SSM casting process atsignificantly reduced retrofitting costs by using the teachingsdescribed herein to cool hypoeutectic Al—Si alloys to the SSM rangerather than with additional abovementioned apparatuses.

FIG. 1 is a graphic representation of a squeeze casting process inaccordance with one embodiment of the invention used for squeezecasting. Persons of ordinary skill will recognize that alternateembodiments are also possible within the scope and spirit of the presentinvention, and that therefore, the invention should not limited to thedetails of the construction or the arrangement of the componentsdescribed herein. According to the embodiment in FIG. 1, a shot sleeveon a casting device first reaches a pour position thereupon initiating apour cycle. The shot sleeve is a receptacle to contain measured amountsof liquid/slurry material to be later transferred into a die cavity.Solid chunks of the cooler hypoeutectic alloy are added to the shotsleeve. Thereafter, molten metal of the hotter hypoeutectic alloy ispoured into the shot sleeve and mixed with the solid chunks. Thecombination in this embodiment leads to rapid dissolution of the solidmaterial into the molten metal and in so doing, drops the initialtemperature of the molten metal. Once in the SSM range, the slurry isthen injected, by any one of a variety of methods known in the art, intothe die cavity and proceeds to be cast.

As mentioned above, the growth of Al particles in the semi-solid phaseis directly correlated to the initial temperature and the time ofcooling of the alloy before casting. The longer an alloy remains in thesemi-solid phase, the likelihood for undesirable growth of large Alparticles is increased. Alternatively, shortening the time an alloyspends in the SSM phase before casting minimizes the growth of large Alparticles by maximizing the number of nucleating events, producing moreAl particles of smaller size. FIG. 2 is representative of themicrostructure of products cast by the inventive steps described.

FIG. 2 shows the microstructure of cast alloys after they have beenquenched. In the particular embodiment presented, a 357 alloy(commercially available alloy of approximately 7% Si) was heated to 640°C. and then combined with 357 alloy chips at room temperature. The 357alloy chips were about 0.25 in³ in average size. The combined liquidmixture cooled to 587° C. by virtue of mixing of the two alloys ofdifferent temperature, before it was finally quenched. Three separatecross sections of the cast product were taken: the edge, mid-radius, andcenter. Microanalysis of the various sections of the castingdemonstrates that the primary Al particles are relatively evenlydistributed with minimal aggregate formation. The Al particles are seenas the light colored particles in the microstructure, and the backgroundis the eutectic (i.e., a mixture of Al—Si). The Al particles shown rangein size from about 40 microns to about 60 microns in diameter from thecenter of the cast though to the edge of the cast. The compactness ofthe Al particles can be assessed relative to a perfectly sphericalparticle and expressed as a ratio of (2πr)²/4πr². Accordingly, aperfectly spherical Al particle would have a ratio of 1 and would appearas a circle on a micrograph, and larger ratios would indicate deviationstherefrom. The compactness ratio of the center of the cast ranged fromabout 1.6 to 1.8 while the edge of the casting ranged from about 2.2 toabout 3.0.

Analysis of the edge cross sections of FIG. 2 shows the morphology ofprimary Al to be less uniform and slightly radiating from a given point(star-shaped). This is generally observed at the outer edges of acasting where the molten liquid or slurry comes in direct contact withthe cold surface of the die cast.

A more rapid drop in temperature results in greater nucleating eventsthan if the temperature is dropped gradually. This has the desirableeffect of generating multiple Al particles that are smaller in size(width and length), but also generally uniformly distributed through outthe alloy. The even distribution of the Al particles, as seen in FIG. 2,allows for better prediction ofinechanical properties with lesslikelihood of mechanical failure which in effect limit the averagegrowth of the Al particles and diminished the likelihood of globularaggregates.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirits and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

1-17. (canceled)
 18. A semi-solid metal product, comprising: the productmade by heating a first aluminum-silicon hypoeutectic alloy to a liquidstate, combining the first alloy and a second aluminum-siliconhypoeutectic alloy at different temperatures to form a semi-solid metal,cooling the semi-solid metal for a length of time effective to increasenucleation of primary aluminum particles therein, and casting thesemi-solid metal.
 19. The product of claim 18, wherein the product isalso made by choosing the length of time to be effective in restrictinggrowth of a primary aluminum phase in the semi-solid metal.
 20. Theproduct of claim 18, wherein the product is also made by combining athird aluminum-silicon hypoeutectic alloy with the first and secondalloys.
 21. The product of claim 18, wherein at least one of the firstand second alloys comprises from about 6 to about 8 percent silicon. 22.The product of claim 21, wherein at least one of the first and secondalloys comprises about 7 percent silicon.
 23. The product of claim 18,wherein the product is also made by heating the second alloy beforecombining it with the first alloy.
 24. The product of claim 23, whereinthe product is also made by heating the first alloy to a highertemperature than the heated second alloy.
 25. The product of claim 23,wherein the product is also made by heating the second alloy to atemperature from about 22° C. to about 660° C.
 26. The product of claim18, wherein the product comprises aluminum particles having an averagediameter from about 40 microns to about 60 microns.
 27. The product ofclaim 18, wherein the product is also made by heating the first alloy toa temperature from about 577° C. to about 715° C.
 28. The product ofclaim 27, wherein the product is also made by heating the first alloy toa temperature from about 577° C. to about 580° C.
 29. The product ofclaim 27, wherein the product is also made by heating the first alloy toa temperature from about 690° C. to about 715° C.
 30. The product ofclaim 27, wherein the product is also made by heating the first alloy toa temperature of about 640° C. and squeeze casting.
 31. The product ofclaim 18, wherein the product comprises aluminum particles having acompaction ratio from about 1.6 to about 3.0.
 32. The product of claim31, wherein the product comprises aluminum particles having a compactionratio from about 1.6 to about 1.8.
 33. The product of claim 18, whereinthe product is also made by choosing the difference in temperaturebetween the first alloy and the second alloy to be effective inproducing the product comprising more homogeneous distribution ofaluminum particles as compared to aluminum particles in a cast productmade by traditional casting methods.
 34. A cast product, comprising: analloy comprising: a first aluminum-silicon hypoeutectic alloy heated toa liquid state; and a second aluminum-silicon hypoeutectic alloy mixedwith the first alloy; wherein the first alloy and the second alloy areallowed to cool for a length of time effective to increase nucleation ofprimary aluminum particles.
 35. The cast product of claim 34, whereinthe length of time is effective to restrict growth of a primary aluminumphase.
 36. The cast product of claim 34, where in the aluminum particleshave an average diameter from about 40 microns to about 60 microns. 37.The cast product of claim 34, wherein the aluminum particles have acompaction ratio from about 1.6 to about 3.0.
 38. The product of claim37, wherein the aluminum particles have a compaction ratio from about1.6 to about 1.8.
 39. The product of claim 34, where in the method iseffective to produce a cast product comprising more homogeneousdistribution of aluminum particles as compared to aluminum particles ina cast product made by traditional casting methods.